1. Field of the Invention
The present invention relates to a differential comparator circuit, a test head, and a test apparatus. More particularly, the present invention relates to a differential comparator circuit that is mounted on a test apparatus for testing a device under test outputting differential signals.
2. Description of Related Art
FIG. 1 shows a configuration of a test apparatus 10 according to a conventional art. The test apparatus 10 includes a subtraction circuit 14, an H side comparing circuit 16, and an L side comparing circuit 18. The subtraction circuit 14 amplifies differential signals output from a device under test (hereinafter, referred to as DUT) 12 with an amplification factor α, and takes and outputs a difference. Then, the H side comparing circuit 16 compares the signals output from the subtraction circuit 14 with an H side threshold value (VrefH) to output a comparison result. Moreover, the L side comparing circuit 18 compares the signals output from the subtraction circuit 14 with an L side threshold value (VrefL) to output a comparison result. In this manner, the test apparatus 10 measures a cross-point of the differential signals output from the DUT 12 by providing the subtraction circuit 14. When the amplification factor α is constant in such a subtraction circuit 14, it is possible to measure the cross-point of the differential signals output from the DUT 12.
FIG. 2 shows a first example of a configuration of an amplifying circuit included in the subtraction circuit 14 according to a conventional art. When a gain G is very large in the amplifying circuit shown in FIG. 2, the amplification factor α=−ROUT/RIN, and thus nonlinearity of the gain G can be ignored. However, it is difficult that such an amplifying circuit increases a gain to realize speedup.
FIG. 3 shows a second example of a configuration of an amplifying circuit included in the subtraction circuit 14 according to a conventional art. Although the amplifying circuit shown in FIG. 3 can realize speedup, a gain is small. Therefore, since nonlinearity of the gain becomes large, it is impossible to hold an amplification factor constant.